Already known known is an exhaust pipe for a turbine, disposed directly downstream of the blades of the runner of the last stage of the turbine.
The walls of the housing of the exhaust pipe define the flow duct shaped as an axial-radial diffuser. The flow duct has mounted therein two stationary deflectors or baffles arranged coaxially with each other.
The baffles are intended to direct the working fluid from the working blades of the last stage of the turbine toward the outlet section of the exhaust pipe. Each baffle has a side surface shaped as a surface of a body of revolution. The longitudinal axis of the baffles is aligned with the axis of the turbine.
The baffles are arranged so that their inlet portions have a radii shorter than the outer radius of the working blades of the last stage of the turbine, with the axial spacing of the inlet portions of the baffles from the outlet or downstream edges of the working blades of the last stage of the turbine being substantially in excess of the axial thermal expansion of the turbine (cf. "Steam Turbines," ENERGIA Publishers, Moscow, USSR, 1976, pp. 302-307).
The known exhaust pipe does not provide for stable performance of the last stage of the turbine when the latter's operating duty is considerably different from the nominal one, i.e. the duty for which the turbine is designed to yield the maximum in economy ratings.
With the flow rate by volume of the working fluid, (e.g. steam), and/or with the angular speed of the rotor varying, the flow coming through the inlet of the pipe is a swirling one. The more the circumferential and radial components of the velocity of the working fluid therein are increased, the more the operating duty of the turbine differs from the nominal one. The centrifugal forces in the swirling flow form the surface of the stream of the working fluid into a hyperboloid of revolution; the same phenomenon is enhanced by a diffuser effect due to the curving shape of the inlet portions of the diffusers. Consequently, the flow of the working fluid breaks off the internal wall of the exhaust pipe, with the outer passages defined by the baffles and the external wall of the pipe housing passing the major share of the flow rate by volume of the working fluid, whereas circulation zones are formed adjacent to the internal wall of the housing, i.e. zones where the working fluid flows along closed surfaces.
Because of the axial asymmetry of the exhaust pipe, the circulation zones are disposed likewise asymmetrically relative to the axis of the turbine; in some areas they extend into the runner of the last stage. Under these conditions, each blade of the runner successively passes through areas with varying duties of the working fluid flow during each revolution of the rotor, which results in varying bending efforts, and, hence, in varying strain of the runner blades, whereby more often than not premature wear of the blades occurs. Furthermore, additional energy losses develop in the runner because of radial overflows and of the unstable character of the working fluid flow.
Additionally, the circulation zones in the exhaust pipes of steam turbines cause moisture particles, i.e. water droplets to find their way from the outlet portion of the pipe toward the runner blades. The resulting erosion wear of the downstream edges of the runner blades reduces the cross-sectional areas of the blades and distorts the shape of these cross-sectional areas, whereby the reliability and economy of the turbine are further impaired.